问题描述
我只是在学习F＃，而在玩游戏时，我注意到如果我更改此代码：
I'm just learning F#, and while playing at tryfsharp.org I noticed that if I change this code:
[0..100]
|> List.sum
至
["A"; "B"; "D"]
|> List.sum
我得到以下错误：
The type 'string' does not support the operator 'get_Zero'
$ b b
（ ，
当我检查;它说该类型必须有一个名为Zero的静态成员。这似乎解释了错误;除了我不能看到任何成员名为Zero on int！
When I checked the definition of List.sum; it says that the type must have a static member called Zero. This seems to explain the error; except for the fact that I can't see any member called Zero on int!
因此;这个零成员适用于ints在哪里？如果我输入 int。，也不能在，这说明int只是一个.NET System.Int32（使用GenericZero：
Digging in F# source code, List.sum (and Seq.sum) is using GenericZero:
let inline sum (source: seq< (^a) >) : ^a =
use e = source.GetEnumerator()
let mutable acc = LanguagePrimitives.GenericZero< (^a) >
while e.MoveNext() do
acc <- Checked.(+) acc e.Current
acc
另一方面，F＃编译器在查询零成员之前构建一个表以查找所有内置数值类型的零值。相关位位于中，
On the other hand, F# compiler builds a table to lookup zero values of all built-in numeric types before querying Zero members. The relevant bits are in this line and the code fragment below.
type GenericZeroDynamicImplTable<'T>() =
static let result : 'T =
// The dynamic implementation
let aty = typeof<'T>
if aty.Equals(typeof<sbyte>) then unboxPrim<'T> (box 0y)
elif aty.Equals(typeof<int16>) then unboxPrim<'T> (box 0s)
elif aty.Equals(typeof<int32>) then unboxPrim<'T> (box 0)
elif aty.Equals(typeof<int64>) then unboxPrim<'T> (box 0L)
elif aty.Equals(typeof<nativeint>) then unboxPrim<'T> (box 0n)
elif aty.Equals(typeof<byte>) then unboxPrim<'T> (box 0uy)
elif aty.Equals(typeof<uint16>) then unboxPrim<'T> (box 0us)
elif aty.Equals(typeof<uint32>) then unboxPrim<'T> (box 0u)
elif aty.Equals(typeof<uint64>) then unboxPrim<'T> (box 0UL)
elif aty.Equals(typeof<unativeint>) then unboxPrim<'T> (box 0un)
elif aty.Equals(typeof<decimal>) then unboxPrim<'T> (box 0M)
elif aty.Equals(typeof<float>) then unboxPrim<'T> (box 0.0)
elif aty.Equals(typeof<float32>) then unboxPrim<'T> (box 0.0f)
else
let pinfo = aty.GetProperty("Zero")
unboxPrim<'T> (pinfo.GetValue(null,null))
static member Result : 'T = result
也就是说，如果你想在用户定义的类型上使用 List.sum ，你需要明确定义零成员。注意， Zero 在字符串类型的情况下没有多大意义。
That said, if you would like to use List.sum on user-defined types, you need to define Zero member explicitly. Note that Zero doesn't make much sense in case of string type.
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